Establishing precise neuronal connections requires targeting of axons and dendrites not only to specific regions or laminae in the brain to find correct target cells but also to specific subcellular domains of dendrites or axons of target cells. Such subcellular specificity of neuronal connections has profound impact on neuronal activity and behavior output. Although significant progress has been made toward understanding mechanisms regulating subcellular specificity of axon targeting, how dendrites are targeted to specific subcellular domains of axons to form synaptic contacts has never been studied. In the Drosophila mushroom body (MB), the olfactory-associative learning and memory center, individual MB output neurons (MBONs) target their dendrites to specific segments (or compartments) of MB axonal lobes to form synaptic contacts. A total of 34 MBONs of 21 types elaborate their dendrites in 16 compartments that together tile the entire MB axonal lobes without overlap. Meanwhile, different types of dopaminergic neurons project their axons to specific compartments shared by MBON dendrites to modulate the synaptic transmission from MB neurons to MBONs by forming synaptic contacts with both MB axons and MBON dendrites. In this proposed project, we will use the MBONs as a novel model system to elucidate cellular and molecular mechanisms governing subcellular- specific targeting of dendrites. We hypothesize that individual types of MB neurons provide attractive cues to direct the targeting of MBON dendrites to specific MB axonal lobes, whereas repulsive interactions between neighboring MBON dendrites and/or adhesive interactions between MBON dendrites and axons of dopaminergic neurons restrict the targeting of MBON dendrites to specific compartments of the MB axonal lobes. We will test this hypothesis and accomplish the objective by pursuing following two specific Aims. Aim 1. We will define cellular mechanisms of subcellular-specific targeting of MBON dendrites. We will investigate the role of MB neurons, dopaminergic neurons, and MBON neurons in regulating the subcellular-specific targeting of MBON dendrites. Aim 2. We will identify candidate molecules that mediate the subcellular-specific targeting of MBON dendrites. We will determine whether and how Ephrin regulates the subcellular-specific targeting of MBON dendrites. Furthermore, we will perform an RNAi knockdown screen to identify novel candidate molecules and pathways that could potentially regulate the subcellular-specific targeting of MBON dendrites. Using the MBONs as a novel model system, we expect to gain mechanistic insights into subcellular specificity of dendrite targeting, which may help us better understand pathogenesis of various developmental neurological disorders and mental diseases caused by defects in dendrite targeting.